With well-established power distribution systems (PDSs) already in place throughout much of the world, an efficient power line communication system (PLCS) could provide more users with high-speed telecommunications access with the minimum investment of “add-on” devices.
The infrastructure for providing broadband Internet access is presently insufficient to meet demand. A power distribution system (PDS), however, could be an ideal vehicle for carrying communications signals in order to meet this demand. Development of a power line communication system (PLCS) would therefore provide more users with high-speed telecommunications access. Since the PDS is already built, the time required to implement a PLCS would be minimal.
Of course, there are a series of problems to be overcome before a PDS can be used as an efficient, high-speed power line communications medium. The following issues, while not exhaustive, are representative considerations of what such a system would require in order to use an existing PDS to transport communication data: a sufficient signal to noise ratio; non-disruptive installation of the “add on” device; safety means such that users and circuitry are protected and isolated from stray current; a signal carrier with a frequency sufficient to support high data transfer rate (e.g. 10 Mbps); means for the data signal to bypass a distribution transformer without loss; bi-directional data transmission; coupling devices that do not interfere with data signal handling; an independent power source for electronic conditioning circuitry at power line interfaces; a power line interface that is impervious to extreme environmental conditions; and means for the data to be readily routed to intended locations without loss.
Given the advantages of being able to use the existing PDS for high-speed data communication, an effective method is required to couple and decouple the signals onto and off of a high or medium voltage power line. The coupling and decoupling of the data signal must be at a level sufficient to maintain an adequate signal to noise ratio in order to discern between the data signal and noise or interference on the line. For any method developed, a significant challenge lies in being able to mitigate the adverse effects of the high voltage 50–60 Hz power signal might have on the communications signal. Additionally, safety from high voltage is of concern.
Whyte, et al. in U.S. Pat. No. 4,142,178 observe: “The use of the distribution network conductors for the transmission of carrier communication signals presents many problems not encountered in high voltage transmission line communication systems. Some of these problems include the poor high frequency impedance characteristics and the high level of electrical noise present on the distribution network conductors which, along with the plurality of distribution transformers and power factor correction capacitors attached to the distribution network, rapidly attenuate the communication signals.”
Whyte teaches using a direct circuitry from a line coupler to a remote data terminal thus bypassing the PDS transformer, which is the primary source of data attenuation. The main use for the transmission of communication signals addressed by Whyte was to perform distribution functions such as automatic reading of utility meters and selective load control. Those functions are still desirable, but the function of high speed, high bandwidth communication transmission preclude direct connection from a transformer to remote data terminals economically.
Use of a low voltage power distribution system as a data communications carrier within a premise is well known. Abraham, U.S. Pat. No. 6,014,386 teaches a communications network within a building using the AC wiring as the infrastructure of the network. Different types of appliances using digital signals may be included within the network. The Abraham patent uses an impedance matching scheme to direct a specific signal to a specific location. Couplers at a control location have unique impedances that are matched by corresponding couplers elsewhere within the building. Thus, specific signals will be de-coupled based an impedance match. Abraham also teaches the use of dielectric inductors in circuit with capacitors to tune the impedance characteristics of couplers.
In a similar manner, Abraham in U.S. Pat. No. 5,625,863 teaches the distribution of multiple video signals distributed within a building using the building's AC wiring as the distribution system. Unique impedance settings direct the signals to unique locations. Abraham in U.S. Pat. No. 5,818,127 describes a distribution system for FM signals within a building by use of the building's AC wiring.
Abraham in U.S. Pat. No. 5,717,685 describes the coupling of data signal on and off a building's AC wiring infrastructure. His invention uses capacitive circuits in serial connection. The circuitry also includes air-core transformers. This arrangement allows impedance tuning of the specific couplers. While Abraham claims a system with a fiber optic source for an input signal in his U.S. Pat. No. 6,014,386, there is no description as to the use of fiber optic isolators.
Abraham also states that the utility firm may use the communications system to communicate utility meter information over the PDS.
Methods for avoidance of distribution transformers are well known. Perkins in a series of patents including U.S. Pat. No. 4,473,816 teaches a communications signal bypassing a multi-phase power transformer where the signal uses the PDS as a carrier. The signal is bi-directional and uses conductive material to affect the bypass. The invention uses multiple capacitors in parallel to neutralize the coupling impedance. Further, the winding ratio, R, between the primary and secondary windings ratio is maintained in the signal frequency across the signal bypass. Signal carrier frequency is in the 3–10 KHz range. Similarly, Perkins in U.S. Pat. No. 4,473,817 teaches a communications signal bypassing a single-phase power transformer.
Kennon, U.S. Pat. No. 4,644,321 uses a non-intrusive coupler to capture the data signal. Kennon teaches the use of a toroid having a multiplicity of turns of a conductor that is in circuit with an amplifier and receiver. The toroid core is non-conductive. The signal thus inductively de-coupled is amplified and used for a load management and filed configuration utility terminal. The system requires a battery for circuitry management.
Brown, U.S. Pat. No. 5,949,327 teaches the use of transformer bypass by coupling using capacitors connected to the primary and secondary terminals of the step transformer. Brown recognizes the need for multiple couplings at different points within the EDN (Electrical Distribution Network or, as referred to in the present description as PDS). Brown also teaches that the communication system use a high frequency signal carrier technique such as CDMA.
Moore, U.S. Pat. No. 5,210,519, describes a communication system that couples data signal from a transmission source using an inductor and de-couples the data at the receiver. This methodology is applied in a closed network and requires selective de-coupling as opposed to routing of the signal. Further, Moore teaches the use of a second transformer for reversing any inductor core saturation that may have occurred in the data de-coupling. This method requires time division of the data coupler between data coupling and saturation neutralization.
Dzung, European Patent Application EP948143, describes a high voltage power line communication system that combines multiple source data signals, couples the combined signal onto multiple power lines using capacitive coupling and de-couples and demodulates the signals, separating and converting the signals back to the original form at the receiver.
Power lines can be located in areas with extreme environmental conditions. Thus, the mechanical design must ensure proper operation when exposed to these extreme conditions and also maintain the required level of safety. Furthermore, any methods developed should be designed so as to have minimal impact to service of customers during installation.
As stated above, public safety is an absolute requirement. Any system using the PDS must isolate the end user (and public in general) from exposure to electric current. The PDS steps medium and high voltage power down to low voltage power (approximately in the 100–240 volt range) using transformers. Transformers are designed to filter out and ground high frequency signals as a safety precaution. Since a high frequency signal carrier is the ideal medium for high bandwidth data transfer, a communications data delivery system needs to circumvent the transformer filtration process while preserving safety protection.